Foam Stability and the Stabilising Ability of Surfactants

Stability of foams and other disperse systems holds a central position in colloid chemistry. Nevertheless, many aspects of this problem are not completely clear. There is no theory as yet that can offer a thorough quantitative explanation of the behaviour of foams with time. It is hardly possible to create such a unified theory, since the dominant thermodynamic and kinetic factors of stability can be different for the different foams. 

The contemporary level of knowledge in foam science enables the solution of many problems related to the kinetics of various processes in a foam (such as film thinning and rupture, foam drainage, diffusion, development of film deformation accounting for the Gibbs and dynamic elasticity, etc.) and to establish the equilibrium conditions of the individual foam elements (films and borders). Thus, it allows the qualitative, and sometime semi-quantitative, interpretation of foam stability. 

Definition of the Term Foam Stability and Foaming Ability of Solutions 

Individual structural elements of the foam, such as films and borders, can be under hydrostatic equilibrium and can correspond to a true metastable state. Therefore, when there is no diffusion expansion of bubbles in a monodisperse foam, its state can be regarded as metastable in the whole disperse system. Krotov [5-7] has performed a detailed analysis of the real hydrodynamic stability of polyhedral foam by solving two problems determination of 

The kinetics of foam collapse, i.e. the process of gas and liquid separation, is characterised by the rate of reduction of foam volume with time or by the rate of decline in its height, if the cross-sectional area is equal along the whole foam column. The stability of the foam as a whole can be characterised quantitatively at any moment by the reciprocal quantity of the rate of foam column destruction. Most often, however, the estimation of the stability of the foam column, is expressed by an integral characteristic time of decay of the whole foam column or a part of it. The relation between the internal foam collapse and the destruction of the foam column is discussed in Section 6.5. Sometimes foam stability is considered in terms of foaming ability of the solution. In general the latter characteristic involves the easiness of foam formation, foam volume and stability. Such an interpretation, however, makes this characteristic rather indefinite. For example, Abramson [12] indicates that for the estimation of the foaming ability of surfactants it is necessary to know the quantity and stability of the foam obtained from a particular surfactant as well as the conditions under which the surfactant acts as a foam stabiliser. That is why it has been repeatedly emphasised that foaming ability 

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Chapter 7 will treat the role of black foam films on foam stability and the stabilising ability of surfactants. 

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